1. Field of the Invention
This invention relates to improved methods for forming metal coating on various surfaces, and more specifically, to a method forming a film containing a gold compound or a platinum compound on a semiconductor surface.
2. Prior Art
The prior art is well aware of the use of semiconductor devices to implement electronic systems which semiconductor devices easily surpass the use of vacuum tubes. It is also well known in the prior art that there is a need that such devices possess high switching speeds and/or high current gain characteristics. In response to this need, the prior art has disclosed the use of gold to reduce the recovery time in, for example, diodes. In the past, gold was diffused through the body of the diode, the gold providing additional recombination centers in the silicon material. With the enhanced recombination of the minority carriers, the reverse recovery time of the diode is reduced.
One such method for diffusing metal atoms into silicon for lifetime control is disclosed by Ciccolella U.S. Pat. No. 3,067,485. In the method disclosed by Ciccolella, the silicon semiconductor body is coated with gold or a goldcontaining material, for example, by electroplating. The silicon is then heated at a temperature in the range from approximately 800 degrees centigrade to approximately 1300 degrees centigrade for a period of time sufficient to achieve substantially complete solid solubility of the gold in the silicon. Ciccolella further points out that in certain semiconductor devices it is desirable to limit or localize the gold treatment, and thus reduce the minority carrier lifetime within only a portion of the semiconductor body. This structure is achieved by using suitable masks or by controlling the depth of gold diffusion by limiting the time of diffusion. However, Ciccolella does not specifically describe how such electroplating techniques are employed, but suffice to say, that such electroplating techniques are well known in the art.
Other methods for depositing a metal film on a substrate in general are also well known in the art. For example, Fefferman U.S. Pat. No. 3,653,946 discloses a method for putting down an adherent gold film comprising the steps of mixing a gold resinate with at least two other specific resinates which, upon the decomposition in air or in an oxidizing atmosphere, form inorganic oxides. The resinate mixture is applied to the substrate and the substrate is fired to a temperature sufficient to decompose the resinate mixture to (1) form free gold and the inorganic oxides, (2) combine the inorganic oxides in situ on the surface of the substrate, and (3) combine the free gold with the combined oxides to form an adherent gold layer containing a high percentage by weight of gold. Such a system, however, requires the use of specific resinates and further requires that relatively high temperatures (approximately 2000 degrees F.) be used to form the gold layer.
Yet another method for applying a gold layer to a substrate is disclosed by Itoh et al U.S. Pat. No. 3,661,727. In Itoh et al, the method requires a gold chloride salt to be dissolved into an aqueous solution of water and hydrofluoric acid. When the surface, for example, a semi-conductor surface, is placed in such a solution, the silicon oxide layer on the top of such surface immediately begins to dissolve gradually by the hydrofluoric acid. Once the semiconductor surface is exposed, gold deposits on the surface of the silicon body. It is apparent from such method that very specific etchants must be used which etchants can adversely affect the semiconductor surface. Moreover, such method requires a pretreatment of the semiconductor surface so as to provide a silicon dioxide protective layer prior to the use of gold salt solution. Finally, such method plates on both sides of the wafer because of the immersion step and mechanical scrubbing is required to remove gold from the undesired areas. This method therefore, not only causes severe pitting of the silicon surface, it can also produce an uneven gold surface and requires the additional step of scrubbing.
The present invention is directed towards a method for supplying an easily applied source of metal atoms for diffusion into silicon for lifetime control. The present invention utilizes a specific solution which may be applied by brushing, spraying or spinning and which forms a metal silicate glass film on the surface of a semiconductor device without the need for extensive heating processes or strong etchants. Thus, the present invention provides an economical method for applying a metal-containing compound in the form of a film to a desired surface.